This invention relates to an interference cancellation apparatus, particularly to an interference cancellation apparatus for use in cancelling interfering waves and adaptively equalizing multi-pass distortion, when subjected to broad-band interfering waves on fading diversity channels.
In general, such an interference cancellation apparatus is for use in combination with a diversity receiving system which has a plurality of diversity routes. The apparatus is operable as a decision feedback equalizer (DFE) which is supplied with diversity reception signals corresponding to the diversity routes to produce an equalized signal.
A conventional interference cancellation apparatus comprises first and second transversal filters each of which has a plurality of controllable taps and which filter the first and the second diversity reception signals into first and second transversal filtered signals in accordance with first and second controllable tap gains, respectively, first and second tap gain controllers for controlling the first and the second controllable tap gains with reference to an error signal and a distributed signal distributed in each one of the taps in accordance with a predetermined adaptive algorithm, third transversal filter which has a plurality of taps and which filters a decision signal into a third transversal filtered signal in accordance with third controllable tap gains, third tap gain controller for controlling the third controllable tap gains with reference to the error signal and a distributed signal distributed in each tap in accordance with the predetermined adaptive algorithm, a decision unit for detecting a level of the equalized signal to decide a decision level for the equalized signal and to produce a decision signal representative of the decision level to supply the decision signal to the third transversal filter, an error producing unit for producing the error signal which is indicative of a difference between a subtracted signal and the decision signal and which is supplied to the first, the second, and the third tap gain controllers, a subtractor for subtracting the third transversal filtered signal from a combined signal to supply the subtracted signal to the decision unit and the error producing unit, and a diversity combination unit for combining the first and the second transversal filtered signals to produce the combined signal.
Such a conventional interference cancellation apparatus is disclosed in "MMSE Equalization of Interference on Fading Diversity Channels", Peter Monsen, IEEE Transactions on Communications, VOL. COM-32, No. 1, January 1984.
However, it still remains as an unsolved problem that the conventional interference cancellation apparatus has defects with respect to an algorithm and a speed of the adaptive equalization. In fact, the conventional interference cancellation apparatus is subjected to disturbing or jamming waves sent from a flying object in addition to the interference from adjacent channels.
In such a case, a changing speed of the interference waves becomes larger than that of the fading. As a result, it becomes difficult that removal of the interference and multipath distortion should be followed in accordance with Least Mean Squares (LMS) algorithm.
It is pointed out by Kojiro Watanabe in a reference ICCC Session No. 46.2.1 "Adaptive Matched Filter and its Significance to Anti-Multipath Fading" that the conventional interference cancellation apparatus proposed by Peter Monsen cannot be operable at a high convergence speed.